This invention relates generally to a system for transporting or storing semisolid materials, such as grease or ground or comminuted food products, and fluid materials, such as oil or printers ink, in bulk quantities, and more particularly to a tank adapted for quickly and efficiently unloading semisolid or fluid material contained therein.
U.S. Pat. No. 4,721,235 discloses an earlier tank for bulk transport and storage of semisolid materials. The tank has a follower piston with an expandable rubber seal at one end thereof for sealing the piston relative to the tank while accommodating changes in internal cross-section of the tank, and means, such as a plurality of pads fastened to the piston and extending radially outwardly therefrom, for preventing canting of the piston as it moves within the tank.
Although this tank performs well and the seal provides an effective wiping action of the tank interior, some transported materials may attack or react with and cause deterioration of the seal. In the transportation of printers ink, for example, toluene, contained in the ink, will react adversely with and deteriorate many of the synthetic rubbers commonly used for making seals and, consequently, cause the seal to stick to and/or leave patches of a residue on the tank interior. This reaction and deterioration reduces the life of the seal. Even seals fabricated from much more expensive corrosion resistant elastomeric materials such as fluoroelastomers (e.g., VITON.RTM.) can be adversely affected by some of the solvents used in printer's ink, rust inhibitors, etc. Further, in some applications, a residue formed by the reaction may contaminate the transported material or, at least, necessitate the manual cleaning of the tank interior.
Moreover, some materials to be transported will, after discharge and during return of the empty tank, dry at the line of contact of the seal and the tank interior and bond the seal to the inner surface of the tank. Such bonding is likely to cause the seal to be torn, abraded or otherwise damaged when the piston is reciprocated. Damage to the seal is likely even though the seal cleanly wipes the inner wall of the tank as the piston moves within the tank.